Atomic Charges in Highly Ionic Diatomic Molecules.

Atomic charges were investigated as functions of detectable atomic and molecular constants at equilibrium structures. It was found based upon the variation idea that atomic charges in highly ionic molecules can be expressed as a function of molecular dipole moments, polarizabilities of free cations, and polarizabilities of free neutral atoms of the corresponding anions. The function can be given in the form of classical Rittner's relationship (. , , 1030). For the ground states of alkali halide molecules, the predicted atomic charges are close to an elementary charge and the predicted dipole moments are in good agreement with the observed values; for spin-restricted high-ionic systems such as the lowest Σ electronic states of BN, AlN, GaN, BP, AlP, GaP, BAs, AlAs, and GaAs molecules, the predicted atomic charges are also near 1 and in good agreement with the results of natural population analysis at MRCI/cc-pvqz and HF/6-311+G(3df) levels. Polarizabilities for the lowest quintet states of B, Al, Ga, N, P, and As ions were also obtained based upon high-level ab initio computations. Atomic charges from other related methods are also investigated for comparison. The results demonstrate that high-quality atomic charges can be obtained with detectable variables, such as molecular dipole moment, vibrational frequency, as well as polarizabilities of the related free atoms and ions.